1. Field of the Invention
The present invention relates to a method and an apparatus for measuring waviness of paper. In particular it provides a quantitative measurement of such waviness which may be due to calendering or other operations used in the production, converting or printing of paper products.
2. Description of the Prior Art
It is well known that during the manufacture of printing paper, the latter is subjected to calendering, i.e. the paper is pressed between a series of rolls in order to improve its surface printing quality. When calendering is very intense, such as soft calendering or supercalendering, where a major reduction of the paper thickness is produced, and particularly when a small number of calendering nips is used, this may result in some waviness in the transverse direction of the paper sheet. Some of this waviness, which can also be called fluting, puckering or cockling, results in waves, wrinkles or compression lines that are permanent and will appear in the printed product where they affect the quality of the paper product by interfering with the reflection of light. This is unsatisfactory, since one of the most important features of printing paper is its gloss, namely its capacity to reflect low incidence light. Moreover, some printing processes, such as heatset offset, may aggravate the waviness of the paper because of application of heat and water during the printing operation.
Examination of various samples of soft calendered and supercalendered papers has shown various degrees of waviness, from light to heavy. The analysis of manufacturing conditions of such samples suggests a relationship between the intensity of calendering and the level of waviness. Thus, in order to better understand this phenomenon and quantify the calendering and other processing parameters, the need for a suitable method and apparatus for measuring such waviness has become apparent.
Several prior art references disclose methods and/or apparatus to measure crimp frequency in crimped material, warp measurement of surfaces such as corrugated webs or sheets, fabric extensibility, and roughness on a web of paper and the like.
Thus, U.S. Pat. No. 4,274,746 of Jun. 23, 1981 provides for the measurement of crimp frequency of crimped material by utilizing the pattern of light reflections produced by the waviness of the material.
U.S. Pat. No. 5,339,534 of Aug. 23, 1994 discloses a device to measure warpage of a surface where a dial indicator is mounted in the middle for measuring deflection of a resiliently flexible band which deflects to conform to a warped surface when placed against it.
Canadian Patent No. 1,331,702 of Aug. 30, 1994 refers to an apparatus for measuring fabric extensibility where the fabric is mounted between two clamps on a base with an arm pivotally mounted intermediate the two ends of the base for rotation about a horizontal axis. When a tensile force is applied to the fabric, it produces an angular deflection of the arm about its axis, which is measured to indicate the extension of the piece of fabric.
Also, Canadian Patent Application No. 2,111,842 laid open Jan. 1, 1993 provides for a roughness detector on a travelling surface of a web, such as paper, by directing a beam of polarized light at an angle onto the web, focused by a focal lens to illuminate a spot on the surface and then collecting a specularly reflected component of the light through a second lens focused on the spot. A detector is aligned to receive a portion of the specularly reflected collimated light and generates a signal depending on the intensity of the light to provide an indication of the roughness or smoothness of the surface.
None of these prior art references deal with the measurement of waviness in paper after calendering or the like, as is done pursuant to the present invention, nor can they be adapted for such purpose.